Group III-N compounds, such as gallium nitride (GaN) and its related alloys have seen significant research in recent years due to their applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting and laser diodes. The large bandgap and high electron saturation velocity provided by certain III-N compounds also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at typical growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3). However, a current problem with the manufacture of GaN thin films is that there is no readily available suitable substrate material which exhibits close lattice matching and close matching of thermal expansion coefficients.
SiC is a semiconducting material which provides excellent thermal conductivity, but is expensive and is presently available only in small wafer sizes. Direct growth of GaN on SiC is generally difficult due to poor wetting between these materials. Although buffer layers, such as AlN or AlGaN, can be used to address this wetting problem, such layers increase the resistance between the device and the substrate. In addition, it is very difficult to prepare a SiC layer having a smooth surface. A rough interface with GaN can cause an increase in the defect density of the GaN layer.
Presently, (0001) oriented Al2O3 (sapphire) is the most frequently used substrate for GaN epitaxial growth due to its low price, availability of large-area wafers with good crystallinity and stability at high temperature. However, the lattice mismatch between GaN and sapphire is over 13%. Such a large mismatch in the lattice constants causes poor crystal quality when GaN films are grown directly on the sapphire, due to stress formation and a high density of defects, including such defects as threading dislocations, microtwins, stacking faults and deep-levels. Sapphire is also an electrical insulator. Use of electrically insulating substrates can complicate certain processing by requiring additional processing steps, as compared to a conducting or semiconducting substrate, due to the inability to make an electrical contact through the substrate. Lattice mismatch may have a larger effect on the dislocation densities than thermal mismatch issues.
Silicon is increasingly being used as a substrate for GaN materials. Silicon substrates have been considered for use as substrates for growth of GaN films. Silicon substrates for GaN growth are attractive given their low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of Si wafers promises easy integration of GaN-based optoelectronic devices with Si based electronic devices.
The disadvantages of Si as a substrate for GaN heteroepitaxy include a +20.5% a-plane misfit which initially led to the conclusion that growth of GaN directly on silicon was not feasible. In addition, the thermal expansion mismatch between GaN (5.6×10−6 K−1) and Si (6.2×10−6 K−1) of 9.6% can lead to cracking upon cooling in films grown at high temperature. Thus, direct growth of GaN on substrates including Si has been found to result in either polycrystalline growth, substantial diffusion of Si into the GaN film and/or a relatively high GaN dislocation density (e.g. 1010 cm−2). Moreover, GaN is also known to poorly nucleate on Si substrates, leading to an island-like GaN structure and poor surface morphology. Thus, the quality of GaN films grown on silicon has been far inferior to that of films grown on other commonly used substrates such as sapphire or silicon carbide. Moreover, the growth conditions that have been used for GaN on Si are generally not compatible with standard silicon processes.
Numerous different buffer layers have been disclosed for insertion between the Si substrate and the GaN layer to relieve lattice strain and thus improve GaN crystal quality. However, even when buffer layers are used, typically the effect of the thermal expansion coefficient mismatch is too large to suppress the formation of cracks in the GaN and related other Group III-N films grown. Thin AlN, GaAs, AlAs, SiC, SiO2, Si3N4 and ZnO, boron monophosphide (BP) or low-temperature GaN layers are exemplary buffer layers have been used for GaN growth on Si.
Typically, GaN substrates have dislocation densities around 109 to 1010/cm2. Lower threading dislocation densities around 107/cm2 may be achievable using a technique called pendio-epitaxy or cantilever epitaxy. However, the lowest values for the threading dislocation densities are only obtained over a narrow region or mesa, which is a few micrometers in length. Properties of LED's, including the internal quantum efficiency (IQE) and lifetime, are deleteriously affected due to the presence of these threading dislocations.